[Approach 2] Accumulate tasks in a proxy task runner until thread starts.

content/public/browser/browser_thread_delegate.h

Index: content/public/browser/browser_thread_delegate.h
diff --git a/content/public/browser/browser_thread_delegate.h b/content/public/browser/browser_thread_delegate.h
index ad5fd7e57eba160f8618d46a9823fa5e6bbc60ee..3688e656e73bb119df5271d68c0702b5bfa01bf0 100644
--- a/content/public/browser/browser_thread_delegate.h
+++ b/content/public/browser/browser_thread_delegate.h
@@ -22,9 +22,6 @@ class BrowserThreadDelegate {
   // Called prior to starting the message loop
   virtual void Init() = 0;
 
-  // Called as the first task on the thread's message loop.
-  virtual void InitAsync() = 0;
-
   // Called just after the message loop ends.
   virtual void CleanUp() = 0;
 };